Move is_legacy() macro family from iwl-4965-hw.h to iwl-4965-rs.h

These macros are for driver's rate scaling implementation, and are not
related to hardware or uCode API values (moved from iwl-4965-hw.h).
Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>